1. Field of the Invention
The present invention relates to the image forming equipment of a document copying machine, etc. in which a fluorescent lamp is utilized as the light source, and in particular to image forming equipment which can control the light energy of a fluorescent lamp to the optimal value at all times.
2. Description of the Prior Art
In a document copying machine, for instance, which is one of this kind of image forming equipments, light which is emitted from a fluorescent lamp is reflected to an image surface of a document placed on the document table, and an optical image effected by the reflection light is imaged on a photoconductor. As the surface of the photoconductor is electrified in advance to a fixed potential, an electrostatic latent image which corresponds to the document image is formed, thereby causing this document image to be read. The electrostatic latent image is developed, transferred to a transcribing paper and fixed by a duplicating process which is already known to the public. After that, a required duplicated image can be obtained accordingly.
In this case, the fluorescent lamp is so arranged that the lengthwise direction thereof is placed along with the width direction of the document, and a detector, for instance, a photo diode (light energy detecting means), which can detect the light energy is provided to the end portion of the fluorescent lamp from a point on the space in the vicinity of the fluorescent lamp.
And the light energy of the fluorescent lamp is always measured by the detector and is so controlled through feed-back that the light energy can be kept at a fixed value regardless of temperature change, time variation, etc. of the fluorescent lamp.
An equipment disclosed by the Japanese Laid-Open Pat. Pub. Sho-57-40277 has been known as a document copying machine which is provided with the above functions.
Hereupon, the fluorescent lamp yields a difference in the light energy between the central part and the end portion in the lengthwise direction of the fluorescent lamp on warming up on a low and a high temperature. Namely, the light energy at the end portion of the fluorescent lamp is lower than that at the central portion thereof. For this reason, when the fluorescent lamp is somewhat warmed as result of copying actions, there is almost no difference in respective light energy, thereby causing almost no problem to occur. However, especially in warming up from power OFF to ON of the fluorescent lamp, the light energy may be greatly influenced by the ambient temperature.
And as a detector for detecting the light energy of the fluorescent lamp is provided at the end portion thereof as described above, the light energy of a lower value is accordingly detected on warming up. Furthermore, as the fluorescent lamp is controlled through feed-back according to the lower value, a current which is larger than the necessity is given to the fluorescent lamp. As a result, the central portion of the fluorescent lamp is warmed up earlier than the end portion thereof, thereby causing the light energy at the central portion to be apt to be overshot. Thereafter, the light energy at the end portion if the fluorescent lamp is gradually increased in accompanying with the temperature rise, thereby causing the feed-back current to be decreased, and finally nearly uniform light energy is able to be obtained in the lengthwise direction of the fluorescent lamp.
Thus, it takes a little longer period of time, for instance, about 30 seconds as the stabilization time of the fluorescent lamp after the warming-up thereof. Therefore, if a copying action is carried out in the meantime, such an inconvenience as an overprinted or excessively exposed copying image may occur.